1. Field of the Invention
The present invention relates generally to an organic electroluminescent device driving method in which an organic electroluminescent device is caused to emit light upon supplying electricity between a hole injection electrode and an electron injection electrode in the organic eletroluminescent device, an organic electroluminescent apparatus using such an organic electroluminescent device driving method, and a display device in which a plurality of organic electroluminescent devices are provided. More particularly, the present invention relates to an organic electroluminescent device driving method in which the luminance of light emitted from an organic electroluminescent device can be simply adjusted, and an organic electroluminescent apparatus utilizing such an organic electroluminescent device driving method, and a display device.
2. Description of the Related Art
In recent years, the needs of flat panel display devices, the consumed power and the size of which are less than those of a CRT (Cathode-Ray Tube), which has been conventionally generally employed, have been increased as information equipments are diversified, for example, whereby an electroluminescent device has been paid attention to as one of the flat panel display devices.
The electroluminescent device is roughly divided into an inorganic electroluminescent device and an organic electroluminescent device depending on the material used.
The inorganic electroluminescent device is sued adapted that a high density electric field is generally exerted on a luminescent portion, and electrons are accelerated within the high density electric field to collide with a luminescence center, whereby the luminescence center is excited to emit light. On the other hand, the organic electroluminescent device is such adapted that electrons and holes are respectively injected into a luminescent portion from an electron injection electrode and a hole injection electrode, the electrons and the holes thus injected are recombined with each other in a luminescence center to bring an organic molecule into its excited state, and the organic molecule emits fluorescence when it is returned from the excited state to its ground state.
In the case of the inorganic electroluminescent device, a high voltage of 100 to 200 volts is required as its driving voltage because a high density electric field is exerted, as described above. On the other hand, the organic electroluminescent device can be driven at a low voltage of approximately 5 to 20 volts.
In the case of the organic electroluminescent device, a light emitting device emitting light in a suitable color can be obtained by selecting a fluorescent material which is a luminescent material. It is expected that the organic electroluminescent device can be also utilized as a multi-color or full-color display device or the like. Further, the organic electroluminescent device can emit light at a low voltage. In recent years, therefore, various studies have been conducted on such an organic electroluminescent device.
Developed as the organic electroluminescent device are ones having a three-layer structure which is referred to as a DH structure in which a hole transporting layer, a luminescent layer and an electron transporting layer are laminated between a hole injection electrode and an electron injection electrode, a two-layer structure which is referred to as an SH-A structure in which a hole transporting layer and a luminescent layer abundant in electron transporting characteristics are laminated between a hole injection electrode and an electron injection electrode, and a two-layer structure which is referred to as an SH-B structure in which a luminescent layer abundant in hole transporting characteristics and an electron transporting layer are laminated between the hole injection electrode and the electron injection electrode.
In recent years, the above-mentioned organic electroluminescent device has been utilized for a display or the like. Therefore, a method of adjusting the luminance of light emitted from the organic electroluminescent device has been examined in causing the organic electroluminescent device to emit light upon supplying electricity, i.e., exerting a voltage, between a hole injection electrode and an electron injection electrode in the organic electroluminescent device.
Examples of a method of adjusting the luminance of light emitted from the organic electroluminescent device include a method of changing a voltage exerted between an electron injection electrode and a hole injection electrode in the organic electroluminescent device, and a method of changing the duty ratio of a voltage applied to the organic electroluminescent device, as disclosed in Japanese Patent Laid-Open No. 54835/1996.
When the voltage applied to the organic electroluminescent device is changed to adjust the luminance of light emitted from the organic electroluminescent device, however, the change in the luminance with the change in the voltage becomes non-linear, whereby the luminance of the light emitted from the organic electroluminescent device cannot be suitably adjusted. In a display device such as a display in which a lot of organic electroluminescent devices are arranged in a matrix, the gray level characteristics are degraded, for example.
In the above-mentioned display device, when the voltage applied to the organic electroluminescent device is changed to adjust the luminance of the light emitted from the organic electroluminescent device, voltage drop caused by a resistance component of a wiring portion changes depending on a current flowing through the wiring portion. Thus, the voltage drop is also affected by the voltage applied to the organic electroluminescent device. When the luminance in a certain organic electroluminescent device is changed, therefore, the luminance in the other organic electroluminescent device is also changed.
On the other hand, when the duty ratio of the voltage applied to the organic elctroluminescent device is changed to adjust the luminance of the light emitted from the organic electroluminescent device, the change in the luminance of the light with the change in the duty ratio becomes approximately linear, whereby the luminance of the light emitted from the organic electroluminescent device can be suitably adjusted.
When the duty ratio of the voltage exerted on the organic elctroluminescent device is changed, however, pulse width modulating circuits having a large current-flowing capacity must be provided ahead of and behind the organic electroluminescent device. In a display device such as a display in which a lot of organic electroluminescent devices are arranged in a matrix, the device becomes complicated, for example.
Furthermore, when the duty ratio of the voltage of electricity supplied to the organic electroluminescent device is thus changed, the pulse width is widened as the duty ratio increases. Consequently, the root mean square value of the current of electricity, i.e., the effective value of the current also increases. When the average value of the current in a predetermined time period thus increases, the average value of the voltage drop caused by the resistance component of the wiring portion also increases as the duty ratio increases.
Even in such a case, therefore, when the luminance in a certain organic electroluminescent device is changed, the luminance in the other organic electroluminescent device is also changed, as in a case where the voltage is changed as described above.